RFID systems today are well known and used for tracking, identifying and monitoring a variety of items from shipping containers to library books. A typical RFID system would comprise several RFID Tags and at least one Interrogator. The Interrogator communicates with the RFID Tags by transmitting carrier signals modulated with data to the RFID Tags. The RFID Tags in turn modulates the modulated carrier signals from the Interrogator using modulated backscattering and reflects and replies to the Interrogator.
Commands and data modulated in the carrier signals are sent to the RFID Tag through the antenna via RF coupling. While the RFID Tag's reception of the Interrogator's commands and data may possesses good signal to noise ratio, the Interrogator's reception of the RFID Tag's reply may not be so.
Amplitude of backscattered signals are generally low and depends on the distance from the Interrogator's antenna to the RFID Tag. The amplitude difference may be as disparate as 120 dB below the carrier signal's amplitude. This results in the amplitude of the RFID Tag's reply being in the range of 100 uV on a carrier signal of 60 volts peak to peak. Furthermore, the modulation frequency of backscattered signals are low compared with the frequency of the carrier signal. The modulation frequency of a backscattered signal is typically a few hundred kilohertz for a carrier signal having a frequency of about 13.56 megahertz. These disproportionate differences in amplitudes and frequencies between carrier and backscattered signals have caused problems with data retrieval.
Presently, research and developmental efforts spent on isolating the backscattered signals from carrier signals have yielded results which are far from satisfactory. The attenuation of the large carrier signals inherently reduces the backscattered signals, giving rise to poor noise performance. In attempting to raise the level of the backscattered signals, some manufacturers have resorted to simply increasing the transmission power. This method not only fails to improve the signal to noise ratio of the Interrogator's reception of the RFID Tag's reply but may even run foul of transmission power limitations.
Another concern is the existence of “blackholes” in the activation field of an Interrogator. These “blackholes” are regions in the activation field where the replies of the RFID Tags cannot be correctly received by the Interrogator. As the backscattered signals take the form of Double Side Band Without suppression of the carrier signal, the side bands can interfere with the carrier signal by way of multi-path reflection and phase delays to cause erroneous effects on the received backscattered signal. This may result in data inversion and data fallout. These effects are especially common in Interrogators employing amplitude demodulation techniques such as diode detectors. It is also occurs in single antenna systems as both transmitted and received signals originate from one point. This creates a higher chance for signals to interfere and with only one reference point, higher difficulty in isolating the backscattered signal.
Another influence in the quality of backscattered data retrieval is the noise in the transmitting circuit. Inherent noise in the transmission will often mask the backscattering signals.
The present invention prevents the erroneous recovery of backscattered signals from RFID Tags and provides extremely high signal to noise ratio for the reception of backscattered signals. The present invention is also designed for low inherent noise.
Conventional Interrogators typically are able to communicate with RFID Tags on a single backscattered frequency. Similarly, typical RFID Tags only communicate with Interrogators via single frequency backscattered modulation. Furthermore, there have not been requirements for Interrogators capable of simultaneous communication with a plurality of RFID Tags operating on a plurality of different frequencies. There also have not been requirements for Interrogators capable of communicating with individual RFID Tags operating on more than one backscattered frequency.
The present invention further addresses the future need for Interrogators capable of simultaneously communicating with a plurality of RFID Tags working on a plurality of different frequencies as well as RFID Tags individually operating on more than one backscattered frequency.